Mold clamping system

ABSTRACT

A clamping system for traversing and clamping molds in a molding machine, the clamping system including a toggle-type clamp arrangement defined by a plurality of pivotally connected links. A movable crosshead is carried on a crosshead guide, and a ball screw is non-rotationally secured to the movable crosshead to extend in the direction opposite from the face of a cooperating fixed mold portion. A pair of ball nuts are carried by the ball screw and each of the ball nuts is drivingly connected with a respective servomotor. One of the servomotors is adapted to provide rapid traverse movement for the movable crosshead, and the other servomotor is adapted to provide high torque for high clamping force to hold a pair of mold halves in tightly contacting relationship during the injection into a mold cavity defined by the mold halves of a flowable material under pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mold clamping system, for example fora plastics injection molding machine, in which the clamping systemoperates to move a movable mold section toward and away from astationary mold section to define a closed mold cavity to permit highpressure injection molding of a part made from plastics material. Moreparticularly, the present invention relates to a mold clamping system inwhich the relative movement between the mold sections is performed by apair of electric servomotors, each of which drives a respective ball nutto advance a single, non-rotatable screw that is attached to a movableplaten that, in turn, carries a movable mold section. The servomotorsselectively move the movable mold section into and out of contact with astationary mold section mounted on a fixed platen.

2. Description of the Related Art

Until relatively recently, in commercial practice the mold closing andclamping systems in machines such as die casting or injection moldingmachines were traditionally hydraulically driven, the hydraulic devicesincluding one or more actuators of the piston-cylinder type for rapidtraverse of the movable mold section as well as for applying clampingforces to maintain the mold sections in contact during injection of thematerial into the mold cavity under high pressure. Further, the use ofhydraulic drives extended both to hydraulic-cylinder-operated machines,in which a large diameter movable ram is carried within a large diameterhydraulic cylinder to provide the required high clamping forces tomaintain mold sections in closed condition during an injectionoperation, as well as to toggle-type machines utilizing a series ofinterconnected links or levers to actuate and control the movement ofthe movable mold section.

However, with the advent of more sophisticated electrical controlsystems for monitoring and controlling the various structural elementsof molding machines during the various steps involved in a moldingcycle, it was found that the use of hydraulic motors and hydraulicactuators resulted in a limitation on the degree of control that couldbe obtained by such sophisticated electrical control systems. Forexample, hydraulic-fluid-operated mold clamping systems are not capableof the precision corresponding with the precision attainable withelectrical control systems, principally because of the small, yetmeaningful, compressibility effects of the hydraulic fluid, as well asthe heating and consequent expansion that the hydraulic fluid undergoesduring the course of a number of molding cycles.

Although reasonably good control has been achieved inhydraulically-operated clamp systems, a different drive arrangement mustbe used if it is desired to attempt to reduce further the overallmolding cycle time, which directly influences the cost of the moldedparts. Although electric motor drives for mold clamping systems havebeen suggested in the past, see, for example, U.S. Pat. No. 2,484,712,which issued Oct. 11, 1949 to Jobst, and United Kingdom Patent No.1,136,573, which was published on Dec. 11, 1968, there has not beenwidespread use of electric motor drives. Moreover, each of those patentsdiscloses an electrical drive system in which an electric motor drives amold closing and clamping system through a gear transmission which,because of the accumulation of mechanical tolerances in the severalinterconnected parts, is incapable of sufficiently precise control of amovable platen position. Furthermore, the above-identified UnitedKingdom Patent discloses a system utilizing a mechanical flywheel, whichresults in a system having a great deal of inertia, thereby furtherrendering difficult the precise and rapid control of the mold closingand clamping system.

Accordingly, it is an object of the present invention to provide anelectric-motor-type drive arrangement for the clamping system of amolding machine.

It is another object of the present invention to provide anelectrically-driven mold clamping system in which a pair of servomotorsare provided, one of the servomotors being operable to effect rapidtraverse of a screw connected with a movable platen, while the otherservomotor is operable when the movable platen has reached the limit ofits path of travel, in order to provide a high clamping force in orderto tightly clamp the movable mold portion against the stationary moldportion.

It is a further object of the present invention to provide anelectrically-operated mold clamping system that is capable of precisecontrol to minimize dead time during a molding cycle, and to therebyreduce the overall time for a single operating cycle.

SUMMARY OF THE INVENTION

Briefly stated, in accordance with one aspect of the present invention,a molding machine is provided for molding materials that are injectedinto a mold cavity defined by a pair of cooperating mold portions. Themachine includes a mold clamping system for opening and closing a pairof cooperating mold portions, the clamping system including at least onemovable platen that carries a mold portion toward and away from anopposed mold portion. A platen drive arrangement is connected with themovable platen for moving the movable platen toward and away from asecond, stationary platen. The platen moving arrangement includes ascrew that is secured to the movable platen in non-rotationalrelationship therewith, and a first drive operatively connected with thescrew for rotational engagement therewith for axially moving the screwat a first linear speed. A second drive arrangement operativelyconnected with the screw is provided for rotational engagement therewithfor axially moving the screw at a second linear speed, wherein thesecond drive arrangement is adapted to provide a larger output torquethan is the first drive arrangement. The first drive arrangement isactivated to cause rapid traverse of the movable platen toward and awayfrom the stationary platen, and the second drive arrangement isactivated to cause slow traverse of the movable platen toward thestationary platen to bring the platens into contacting relationship andto hold the platens together tightly with a contact force that issufficient to offset an oppositely directed force resulting from theinjection into the mold cavity of pressurized plastic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an injection molding machine having a moldtraversing and clamping arrangement in accordance with the presentinvention.

FIG. 2 is a side elevational view, partially in section, of theinjection molding machine shown in FIG. 1, in which the upper half ofthe figure shows the parts of the mold clamping system in their relativepositions when the movable platen is in its right-most position with themold sections engaged, and in which the lower half of the figure showsthe respective parts in their relative positions when the movable platenhas been withdrawn from and is fully retracted away from the stationaryplaten.

FIG. 3 is an elevational view taken along the line 3--3 of FIG. 2.

FIG. 4 is an elevational view taken along the line 4--4 of FIG. 2.

FIG. 5 is an elevational view taken along the line 5--5 of FIG. 2.

FIG. 6 is a block diagram showing a control arrangement for theinjection molding machine shown in FIGS. 1 and 2.

FIG. 7 is a sequence diagram for the injection molding machine shown inFIGS. 1 and 2, identifying the several portions of a single operatingcycle of the machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIGS. 1 and 2thereof, there is shown a mold clamping system 10 of an injectionmolding machine 12. Although an injection molding machine includes amold clamping system 10 as well as a plastication and injection system14, only the mold clamping system has been shown in detail because thoseskilled the art are familiar with the possible forms of plastication andinjection systems that can be employed. Each of the mold clamping system10 and plastication and injection system 14 is mounted on a base (notshown). For illustrative purposes, plastication and injection system 14can be a typical reciprocating screw injection unit of a type that iswell known to those skilled in the art.

It should be noted at the outset that although described herein in thecontext of a plastics injection molding machine, the present inventionis also applicable to metal die casting machines, rubber moldingmachines, as well as to other machines wherein rapid movement of onepart relative to another part is desired, along with a high force tohold the two parts in predetermined positions.

Clamping system 10 includes a stationary platen 16 to which is securedone mold section 18 that includes a portion of a mold cavity 20 thatdefines the outer surface of a desired part to be molded. Spaced fromstationary platen 16 along a longitudinal axis is a fixed crosshead 22which, like stationary platen 16, is secured to the machine base (notshown). Extending between stationary platen 16 and fixed crosshead 22are four cylindrical tie rods 24, which are preferably disposed in asquare or a rectangular array with their axes parallel with each other.Stationary platen 16 and fixed crosshead 22 are secured to the tie rodsby means of respective nuts 26 that are threadedly engaged with anexternal thread (not shown) that is formed at the respectivelongitudinal ends of each of tie rods 24.

Slidably carried on tie rods 24 is a movable platen 28, which is ofgenerally rectangular configuration and which includes a face 30 that isopposite to and in facing relationship with a corresponding fixed face32 on stationary platen 16. Face 30 of movable platen 28 carries moldsection 34 that includes another portion of mold cavity 20. Mold section34 is adapted to be cooperatively engaged with mold section 18 to definetherebetween a closed mold cavity 20 of a particular shape to providemolded parts having the desired outer configuration.

As best seen in FIG. 2, fixed crosshead 22 includes a recess 36 thatfaces movable platen 28 and is adapted to receive a movable crosshead 38that is slidably carried on a pair of vertically spaced, parallelcrosshead guide rods 40 (see FIG. 3), each of which has its respectivelongitudinal axis positioned within the rectangular cross sectiondefined by tie rods 24, and disposed in parallel with the longitudinalaxes of tie rods 24.

Movable crosshead 38 includes a vertically-extending center member 42that carries a pair of spaced, parallel sleeves 44 through whichrespective crosshead guide rods 40 pass. Movable crosshead 38 alsoincludes two pairs of laterally spaced upper link supports 46 and twocorrespondingly arranged pairs of lower link supports 48 that extendfrom movable crosshead 38 in a direction toward movable platen 28. Eachof upper and lower link supports 46, 48 includes a respective pivot 50,to which one end of a respective idler link 52 is pivotally connected.Thus, movable crosshead 38 carries four such idler links 52. Theopposite ends of each of idler links 52 are connected with respectivefirst links 54 at an intermediate position between the spaced ends ofeach of first links 54. One end 56 of each of first links 54 ispivotally carried in a yoke 58 that extends outwardly from fixedcrosshead 22 toward movable platen 28. The other, opposite ends 60 ofeach of first links 54 are pivotally connected with one end ofrespective second links 62, the opposite ends 64 of which second links62 are each pivotally connected with movable platen 28 throughrespective pivots 66.

As best seen in FIG. 2, the initial positions of the respective movableparts of clamp system 10 are shown in the lower portion of the figure,wherein movable crosshead 38 is received within recess 36 of fixedcrosshead 22, with respective first and second links 54 and 62 defininga V. As movable crosshead 38 is shifted toward the right, as viewed inFIG. 2, upper and lower link supports 46, 48 carry idler links 52 to theright, thereby pivoting the lowermost first links 54 in acounterclockwise direction about respective pivots 57, and the uppermostfirst links 54 in a clockwise direction about their respective pivots57. The pivotal movements of first links 54 move second links 62 in adirection toward stationary platen 16, as a result of which movableplaten 28 is shifted along tie rods 24 from left to right, as viewed inFIG. 2.

When movable crosshead 28 has moved from left to right the maximumdistance, the positions of the respective parts are as shown in theupper half of FIG. 2, in which the respective longitudinal axes ofconnected ones of first and second links 54, 62 are aligned, with theresult that movable platen 28 is moved to its right-most positionrelative to fixed crosshead 22. As a consequence, mold sections 18 and34 are brought into cooperative relationship and define therebetweenclosed mold cavity 20 that is adapted to receive molten plastic materialfrom injection unit 14, which injects softened and flowable plasticmaterial through a sprue (not shown) that extends from mold cavity 20through the rightmost wall of stationary mold section 18, andcommunicates with passageway 70 (see FIG. 5) in stationary platen 16,with which passageway injection unit 14 communicates.

After the necessary quantity of molten plastic material is injected intoclosed mold cavity 20, and after a sufficient cooling period haselapsed, movable crosshead 28 is retracted by moving it from right toleft, as viewed in FIG. 2, to separate mold sections 18 and 34 asufficient distance to permit removal of the resulting molded part. Inthat connection, an ejector cylinder 72 can be provided on movableplaten 28 to shift an ejector plate 74 that carries one or more ejectorrods (not shown) to eject the molded part from movable mold section 34,whereupon movable crosshead 28 can again be moved toward the right, asviewed in FIG. 2, to position the mold sections for receiving a secondshot of molten plastic material to form a second part.

Referring once again to FIG. 2, and particularly to the lower halfthereof, movable crosshead 28 includes a ball screw 76, that isnon-rotatably secured thereto by means of a nut 78. Ball screw 76 ispositioned to extend from movable crosshead 38 in an opposite directionfrom movable platen 28 and through the transversely extending end wall80 of fixed crosshead 22.

As best seen in FIG. 1, ball screw 76 is driven by a pair of electricservomotors 82, 84 through respective toothed belts 86, 88 that areadapted to drive respective ball nuts 90, 92, each of which is rotatablycarried on ball screw 76 and is adapted to be rotatably engagedtherewith.

As best seen in FIG. 1, servomotor 82 is supported from a housing 94that houses drive sprocket 96, and is secured to rear wall 80 of fixedcrosshead 22. Similarly, servomotor 84 is supported from a housing 98that houses drive sprocket 100 and is also secured to rear wall 80 offixed crosshead 22, on the laterally opposite side of a verticalcenterline passing through ball screw 76. As is apparent from FIG. 1,drive sprocket 96 has a larger diameter than drive sprocket 100 and,consequently, the former causes more rapid rotation of its associatedball nut 90 than does drive sprocket 100 to, in turn, cause more rapidlinear movement of ball screw 76 in a direction based upon the directionof rotation of the ball nut. Thus associated servomotor 82 is adapted toprovide rapid traverse of movable crosshead 38.

On the other hand, servomotor 84, which carries smaller diameter drivesprocket 100, is adapted to rotate its corresponding ball nut 92 at aslower rotational speed, to thereby impart greater torque to ball screw76.

The respective ball nuts 90, 92 are spaced from each other along theaxis of ball screw 76 and are also spaced from rear wall 80 of fixedcrosshead 22 and the inner wall of the housing by means of thrustbearings 102, which can be of any convenient type, as will beappreciated by those skilled in the art. The positions of the respectiveservomotors 82, 84 and the differences in sizes of the respective drivesprockets 96, 100 are also apparent in FIG. 4 of the drawings.

The operation of the respective servomotors 82, 84 is controlled by acontrol system that is schematically illustrated in FIG. 6. As thereshown, each of servomotors 82, 84 is connected to a motor control unit104 that is adapted to operate the respective motors duringpredetermined portions of a molding machine operating cycle.

Because the present invention contemplates very accurate control of thelongitudinal position of the ball screw, an external, linear positiontransducer 106 is provided for determining the position of ball screw 76relative to fixed crosshead 22. Another possible form of ball screwposition sensor can be, for example, an internal angular positionencoder (not shown), and it is possible to provide that encoder ineither servomotor because each of servomotors 82, 84 is normallycontinuously connected with ball screw 76, and therefore either motorcan provide the sensing point for a suitable output signalrepresentative of the longitudinal position of the ball screw.

An operating cycle for an injection molding machine incorporating themold clamping system in accordance with the present invention is shownin FIG. 7, which shows an operating cycle that begins when the moldhalves are separated from each other by a predetermined distance that issufficient to permit a molded part to fall from the mold cavity to asuitable part collector positioned below the mold portions. Initialmovement of movable platen 28 and movable mold section 34 towardstationary platen 16 and stationary mold portion 18 is accomplishedthrough the operation of rapid traverse servomotor 82, referred to inFIG. 7 as "servomotor #1," until a position has been reached at whichfirst and second links 54 and 62 are each in their extended position, asillustrated in the upper half of FIG. 2, and movable platen 28 is sopositioned that movable mold section 34 is either very close to or invery light initial contact with stationary mold section 18. At thatpoint servomotor 84, the high torque clamping servomotor, referred to inFIG. 7 as "servomotor #2," is started to provide a high torque input tothe ball screw. Simultaneously with the startup of high torque clampingservomotor 84 rapid traverse servomotor 82 is dropped off line andremains connected with ball screw 76 to free-wheel during the time hightorque servomotor 84 is in operation. The latter then supplies therequired torque to ball screw 76 to move movable mold section 34 anyremaining distance toward fixed mold section 18 to cause the two moldsections 18, 34 to tightly engage and define closed mold cavity 20. Whenmolten plastic material is injected from injection unit 14 into moldcavity 20 at high pressure, the resulting force of the high pressurematerial acting on the projected area of mold cavity 20 of movable moldsection 34, which force acts in the direction of fixed crosshead 22 andtends to move mold section 34 in a direction away from stationary moldsection 18, is counterbalanced by the opposite force resulting from thetorque provided to ball screw 76 by high torque servomotor 84.

After mold cavity 20 has been filled with the molten material, and aftera sufficient time period has elapsed within which the material insidethe mold has cooled to a sufficient extent that the resulting moldedpart will maintain its shape, clamping 10 unit is opened by initiallyoperating high torque servomotor 84 in a reverse direction, as comparedwith the direction of rotation to move the mold sections together, andupon initial separation of the mold sections high torque servomotor 84is dropped off-line, but remains connected to ball screw 76, and rapidtraverse servomotor 82 is brought on line to rapidly retract movableplaten 28 and movable mold section 34 away from stationary mold section18 until a predetermined distance has been reached, at which pointejector plate 74 is actuated by ejector cylinder 72 to eject the moldedpart from the mold, and thereby permit the mold sections to be broughttogether once again for molding another part during the next moldingcycle.

It will apparent that the disclosed clamp drive system permits veryaccurate control over the position and operation of the movable platen,thereby permitting the machine to be operated so that there is a minimumoverall cycle time, resulting in economically produced parts.

In addition to the applicability of the present invention to a moldclamping system for an injection molding machine, the present inventioncan also be used in other machines in which molds are used to definemolded articles made from other materials, such as die casting machinesfor forming metallic articles, and rubber molding machines for moldingrubber articles. Additionally, the invention can also be used in otherapplications where a low torque is required for a rapid traverseoperation and a high torque is required for a subsequent operation, suchas in a machine tool in which rapid traverse is desired for moving aworkpiece by means of a screw-operated conveying and feeding system tocarry a work holder to which a workpiece is fixed and to move theworkpiece from a loading station to a cutting or forming station, andthen a high feed force is required to feed the workpiece against a fixedcutting or forming tool while the cutting or forming operation isproceeding.

Although particular embodiments of the present invention have beenillustrated and described, it will be apparent to those skilled in theart that various changes and modification can be made with out departingfrom the spirit of the present invention. Accordingly, it is intended toencompass within the appended claims all such changes and modificationsthat fall within the scope of the present invention.

What is claimed is:
 1. A clamping system for a molding machine formolding flowable materials in a mold cavity defined by a pair ofcooperating mold portions, said clamping system comprising:a. moldclamping means for opening and closing a pair of cooperating moldportions, the mold clamping means including at least one movable platenthat carries a mold portion; b. platen moving means connecting with theat least one movable platen for moving the at least one movable platentoward and away from a second platen, the platen moving means includingscrew means secured to the movable platen in non-rotational relationshiptherewith, a first rotary drive means operatively connected with thescrew means for rotational engagement therewith for axially moving thescrew means at a first linear speed, and second rotary drive meansoperatively connected with the screw means for rotational engagementtherewith for axially moving the screw means at a second linear speed,the second drive means adapted to provide a larger output torque thanthe first drive means, wherein the platen moving means includes firstball nut means rotatably carried on the screw means and operativelyconnected with the first drive means, and second ball nut meansrotatably carried on the screw means and operatively connected with thesecond drive means, and wherein the first drive means is activable tocause rapid traverse of the movable platen toward and away from thesecond platen, and the second drive means is activable to cause slowtraverse of the movable platen toward the second platen to bring theplatens into contacting relationship and to hold the platens tightlytogether with a contact force sufficient to offset an oppositelydirected force resulting from the injection into the mold cavity underpressure of a flowable material.
 2. A clamping system in accordance withclaim 1, wherein the second platen is fixed in position relative to themolding machine.
 3. A clamping system as claimed in claim 1, wherein thefirst and second drive means are electric drive motors.
 4. A clampingsystem in accordance with claim 3 wherein the electric drive motors areservomotors.
 5. A clamping system in accordance with claim 1, whereinthe first and second drive means include belt means and sprocket meansextending between the respective drive means and the respective ball nutmeans.
 6. A clamping system as claimed in claim 1, wherein the firstdrive means is adapted to free-wheel while the second drive means is inoperation, and the second drive means is adapted to free-wheel while thefirst drive means is in operation.
 7. A clamping system in accordancewith claim 1, wherein the second drive means has a higher speedreduction drive than that of the first drive means.
 8. A clamping systemin accordance with claim 1, wherein the mold clamping means includes atoggle clamp including a plurality of pivotable links extending betweena movable crosshead and the movable platen.
 9. A mold clamping system inaccordance with claim 1 wherein the first and second ball nut means areadjacent each other on the screw means, and are separated with thrustbearings and include thrust bearings on their respective outwardlyfacing surfaces.
 10. A mold clamping system in accordance with claim 1wherein the first and second drive means are independently operable andeach drive means remains connected with the ball screw means throughouta machine operating cycle.